Reword description of promotability

Author: ecstatic-morse

promotion.md

@@ -85,8 +85,8 @@ contexts are treated as explicit.
 
 ## Promotability
 
-We have defined when it is desirable to promote expressions but have not yet
-defined which expressions can actually be promoted. We refer to such
+We have described the circumstances where promotion is desirable, but what
+expressions are actually eligible for promotion? We refer to eligible
 expressions as "promotable".
 
 ### Named locals